Patent reference 1 discloses a laser radar device that transmits a pulse-shaped transmission light along an optical scan axis, analyzes a Doppler frequency light signal included in received light based on this transmission light, and measures the wind speed and so on in the direction of the optical scan axis. This laser radar device includes an analyzing circuit that converts the Doppler frequency light signal into a Doppler frequency electric signal, and analyzes this Doppler frequency electric signal.
Further, a laser radar device that measures wind directions and speeds uses a light intensity modulation unit within a transmitting unit, in order to pulse transmission light. However, in an acousto optic modulator (AO modulator) that is used for this light intensity modulation unit, carrier leak light resulting from the reverberation of an ultrasonic signal for modulation occurs. This carrier leak light then induces a beat signal unnecessary for an optical heterodyne receiver.
Because this unnecessary beat signal coincides with the Doppler signal to be measured in the frequency domain, there is a disadvantage that it becomes impossible to estimate the Doppler frequency correctly.
As a measure against the disadvantage, in the laser radar device disclosed by patent reference 1, an optical circulator is placed as a stage preceding the input end of the light intensity modulation unit, and a total reflection mirror is placed as a stage following the output end of the light intensity modulation unit. As a result, the light can be caused to make a round trip in the light intensity modulation unit, and the carrier leak light during pulse off time intervals can be reduced.